This invention relates to circumferential flow type liquid pumps, and more particularly to a circumferential flow type liquid pump used as a fuel pump for pumping a liquid-phase fuel such as gasoline from the fuel tank into an internal combustion engine of a vehicle.
FIGS. 6 through 8 are sectional views showing a conventional circumferential flow type liquid pump as disclosed, for instance, by Japanese Published Unexamined Patent Application No. 79193/1985 In these figures, reference numeral 100 designates a pump casing assembly which is made up of a pump casing 110 comprising a pump base 1 and a pump cover 2, and an impeller 3 rotatably supported in the pump casing 110. The impeller 3 has vanes 3A in its outer periphery and is mounted on a shaft 4 so that it is rotated around its central axis with respect to the pump casing assembly 100.
The pump casing assembly 100 defines an arcuate pump flow path 5 elongated along the outer periphery of the impeller 3 and a suction inlet 2A and a discharge outlet 1A which are opened at both ends of the pump flow path 5. The pump flow path 5 receives the vanes 3A of the impeller 3. The pump flow path 5 is made up of recesses 1B and 2B which are formed in the pump base 1 and the pump cover 2.
The end portion of the pump flow path 5 which is on the side of the suction inlet 2A where the internal pressure is low is formed into an arcuate enlarged flow path 5A having a predetermined length which is larger in section than the remaining portion, and has a step 5B at the end where the sectional area is decreased; in other words, the remaining portion of the pump flow path 5 between the step 5B and the discharge outlet 1A is smaller in sectional area than the enlarged flow path 5A, and accordingly higher in internal pressure than the latter. A small hole, namely, a gas venting hole 2C is formed in the enlarged low path 5A near the step 5B so that the pump flow path 5 is communicated with the outside of the pump casing assembly.
The shaft 4 of the rotor 11 of an electric motor 10, coupled to the pump casing assembly 100 is rotatably supported by bearings 12 and 13. The pump casing assembly 100 is coupled to an end cover 14 through the yoke 15 of the motor 10. The end cover 14 has a pump discharge outlet 1A for supplying liquid, for instance, to an engine (not shown). The yoke 15 accommodates the rotor 11, and forms a liquid chamber 16 between the pump casing assembly 100 and the end cover 14 to store a liquid such as a liquid-phase fuel discharged through the discharge outlet 1A. Permanent magnets 17 serving as stator, and brushes 19 in sliding contact with the commutator 18 of the rotor 11 are provided inside the yoke.
The operation of the circumferential flow type liquid pump thus constructed will be described.
As the impeller 3 is rotated clockwise, in FIG. 7, by the electric motor 10, a liquid such as a liquid-phase fuel is sucked into the pump flow path 5 through the suction inlet 2A at the end. The liquid thus sucked is increased in pressure by the fluid friction resistance which is provided by high speed rotation of the vanes 3A of the impeller in the pump flow path 5, so that it is caused to flow clockwise in FIG. 7, and then flow through the discharge outlet 1A at the other end into the liquid chamber 16. In this operation, the vanes of the impeller contact the liquid, so that the liquid is partially vaporized, thus forming bubbles in the liquid. The bubbles thus formed are discharged out of the pump casing assembly 100 through the gas venting hole 2C near the step 5B in the enlarged flow path.
When, in a circumferential flow type liquid pump used as a fuel pump, bubbles are formed in the pump flow path by vaporization of the fuel and stayed therein, so-called "vapor locking" may be caused to obstruct the flow of liquid, thus lowering the pumping capacity greatly. In order to overcome this difficulty, in the above-described conventional circumferential flow type liquid pump, the gas venting hole is formed in the pump cover so that the intermediate portion of the pump flow path is communicated with the outside of the pump casing assembly. Thus, the bubbles formed in the pump flow path by vaporization of the liquid are discharged out of the pump casing assembly through the gas venting hole.
However, since the gas venting hole is formed in the bottom of the enlarged flow path, the liquid friction force is not sufficiently transmitted, and the bubbles formed in the pump flow path when the vanes of the impeller contact the liquid are caused to flow along the inner circular periphery of the pump flow path and near the impeller because of the difference between the bubbles and the liquid both in centrifugal force and in specific gravity, and the liquid friction resistance is further decreased. Thus, it becomes impossible to discharge the liquid and bubbles from near the bottom of the pump flow path into the outside of the Dump casing assembly, and vapor locking may occur.